Gas engines, which are also called gas-fuelled or gas-fired engines, are used to drive pumping stations of natural-gas pipelines, blowers and generators in, for example, purification plants and on gas tankers. Gas engines may be two- or four-stroke, spark-ignited or compression-ignited. Gas Otto engines ignite a mixture of gas and air using spark plugs. Gas diesel engines use a continuous injection of a small amount, such as, for example, 5-10%, of diesel fuel.
Gas engines operate at high temperatures such as greater than 200° C. in a piston environment. These high temperatures cause oxidation of the gas engine lubricating oil composition, which produces undesirable acids. These acids cause corrosion of the gas engine, in particular, corrosion of bearings in crankshaft journals and crankpins.
Gas engine oils are formulated to minimise engine wear, particularly wear resulting from such acid corrosion.
EP-A-1 347 034 (“'034”) describes gas engine lubricating oil compositions having a boron content of at least 95 ppm and comprising at least one metal salicylate having a TBN of 60 to 140. Compositions exemplified in '034 have a low sulphated ash content (not more than 0.6 mass %), include borated dispersant and alkylated dispersant and alkylated diphenylamine anti-oxidant components, and have a boron content of 105 ppm. It is however found that compositions such as those of '034 exhibit adverse lead corrosion properties.